Manage your subscription

Birdwatching in stereo captures flocks in 3D

By Colin Barras

Using two digital camera to create a stereo vision system, physicists can know the 3D position of 90% of birds in a flock

(Image: PNAS)

Two consumer digital cameras, some fishing line and a spot of number crunching have made it possible for researchers to track the 3D positions of more than 2000 individual starlings flying in a single tightly-packed flock.

Previously, scientists could only work out the positions of tens of birds at any one time. Unveiling the secrets of such complex group behaviour could have applications in economics.

Because of the difficulties in tracking individual birds in large flocks, biologists have instead used computer simulations to study flocking.

Advertisement

“Models can complement empirical investigation but they cannot replace it,” says Andrea Cavagna at the Italian National Institute for Condensed Matter Physics. His stereo camera system now makes it possible to gather data from real flocks.

Stereo vision

Cavagna’s STARFLAG (starlings in flight) research team did their birdwatching from the top of the National Museum of Rome, Italy.

They photographed a starling flock using a pair of commercially-available digital cameras positioned 25 metres apart, mimicking the depth perception of animals with stereo vision.

By comparing images taken at precisely the same moment from the two slightly different positions they could plot the 3D position of each bird in the flock.

For the results to carry any meaning, Cavagna’s team had to carefully align their cameras. They clamped each onto a short pivoting boom and used a 25m length of fishing line to position the booms at the correct angles.

Blobby birds

They developed software to isolate the dark birds from the pale sky behind, and to count the number of birds in larger blobs that represent two or more overlapped birds.

But the real novelty is the method used to match up the same birds on the two images, says biologist Frank Heppner at the University of Rhode Island, US. “If the individual birds are different colours or sizes, there’s no problem,” says Heppner. “Alas, starlings are identical.”

Cavagna uses what is called the epipolar technique. After isolating a bird in one image, his software estimates where it should appear in the second image, before homing in on the most likely match and calculating its 3D position.

“Cavagna is absolutely the first person to use this technique with birds,” says Heppner.

Birds of a feather

Cavagna’s technique can work out the position of 90% of the birds in a flock, and has successfully processed a flock of 2700 in under 3 hours. By comparison, working out the positions of just 16 without automated analysis took 1000 hours, says Heppner.

Cavagna hopes that the mechanisms underlying flock movements may also apply to human economic behaviours, which also exhibit flocking phenomena like passing fads.

But the new method has not been met with universal praise by biologists. “If you’ve been working on bird behaviour for 20 years and then some guy – a physicist at that – turns up with a better result, you’re going to be pissed off,” says Cavagna.

He and some biologists, including Heppner, are launching a charm offensive to get the technique accepted by the biological community.

“I cheerfully admit that it would take a lot more distance for me to understand the physics here than for a physicist to understand the biology,” Heppner says.

“On the other hand, I’ve been watching these damned flocks for almost fifty years, and I can think like a starling. I challenge a physicist to do that.”